Technical Abstract:
Prompted by the emerging bioenergy industry, conversion of land use in agricultural landscapes is expected to alter belowground carbon (C) dynamics leading to widespread changes in soil C storage. ‘Marginal’ areas not well suited for annual row crops have been targeted for production of bioenergy crops such as perennial grasses due to substantial productivity and the numerous environmental benefits derived from C storage. The physical protection of organic matter (OM) within soil aggregates occurs rapidly in response to changes in land use and soil management. Mechanistic understanding of the belowground processes that lead to C storage has shown the importance of roots and microbes for soil aggregation. These effects have largely been quantified under stable soil conditions, however, and the relative importance of variation in soil conditions on belowground C cycling processes—including root characteristics, microbial biomass C and N, and extra-cellular microbial enzyme activities—remain largely unknown. We used data on 11 soil properties, three root characteristics, and activity of four microbial enzymes to determine influences on changes in physically protected C pools over three years in a bioenergy cropping system (continuous corn, triticale/ sorghum, switchgrass) study. Results from structural equation modeling analyses show that differences in root biomass of cropping systems and variation in soil texture across a heterogeneous agroecosystem had an overall impact on short-term C cycling processes that led to shifts in stored OM pool changes over time. Specifically, decreased soil sand content and greater root biomass positively impacted aggregation change (R2=0.942) over time, driving increases in physically protected C within soil aggregates (R2=0.510). Our model suggests that 1) the influence of root biomass on aggregation shown in other studies may be mediated by the activity of microbial enzymes involved in C and N processing (beta and N-acetyl glucosaminiases, cellobiohydrolases, beta xylosidases); and 2) variation in soil texture exerts a significant effect on enzyme activities that impact physical protection of C. Overall, our results demonstrate that variation in soil texture across heterogeneous landscapes may strongly influence the expectations of C storage rates bioenergy production systems, and that these appear to be due to strong affects on soil microbial enzyme activities. Our results improve the mechanistic understanding of belowground C cycling which may advance process-based models of soil C dynamics and allow for more accurate scaling of soil C data under future land-use scenarios.